Unbalanced current on neutral formula

[bbuckway]

Can anyone help with how to figure what the unbalanced current would be on a Wye system neutral?
For example:
Phase A is 130 amps
Phase B is 110 amps
Phase C is 90 amps
What would the amps be on the neutral.

 

[jumper]

Here is a good thread with the formula.

http://forums.mikeholt.com/showthread.php/110225-Neutral-Current-Formula?

 

[david luchini]

There isn't really enough information to accurately determine the neutral current in your question. You need to know the phase angles of the 3 currents relative to each other.

The formula in the thread linked to by Derek would work if the phase angles of the 3 currents were 120 degrees apart.

 

[kingpb]

And if their not 120 deg apart, then you have to use symmetrical components.

 

[sgunsel]

What's wrong with 120 degrees for 3 phase? What else do you have?

 

[Smart $]

Can anyone help with how to figure what the unbalanced current would be on a Wye system neutral?
For example:
Phase A is 130 amps
Phase B is 110 amps
Phase C is 90 amps
What would the amps be on the neutral.

If you have Excel, download my calculator in the thread linked to earlier (let me know if the link does not work).

Anyway, your question leads me to ask if there is an ulterior purpose for asking? Mostly, are you using this to determine a size for a neutral conductor. If so, it's not how you determine the size.

 

[david luchini]

What's wrong with 120 degrees for 3 phase? What else do you have?

I'm not sure I understand your question. There's nothing wrong with 120 degrees for 3 phase. But you have what ever the load power factor gives you. Unless the power factor for each load is the same, you won't have 120 degree phase separation on the 3 line currents.

 

[Smart $]

And if their not 120 deg apart, then you have to use symmetrical components.

Says who?

 

[charlie b]

And if their not 120 deg apart, then you have to use symmetrical components.

Says who?

Well, there is another way. You can use trigonometric identities and solve it long-hand.

 

[david luchini]

Well, there is another way. You can use trigonometric identities and solve it long-hand.

This is my preferred method. I find it simple and straight forward.

 

[kingpb]

I just model it and let ETAP figure it out! :happyyes:

 

[Besoeker]

What's wrong with 120 degrees for 3 phase? What else do you have?

The voltages are 120 degrees apart. The currents might not be.